Originally Posted by laoshun
usually shrimp swim around near bettas at the surface searching for food.
My red cherry shrimp stay the heck away from my betta (or they're lunch) and forage mostly at the bottom, in the various hidey-places I provide for them.
There's a few aggressive shrimp species (the long-armed shrimp is famous for eating small fish) but most really aren't - I'd suspect your fish of tail-biting himself before I'd blame the shrimp unless that one larger shrimp proves to be a highly carnivorous species.
Maybe take it back to the store and ask for it to be identified? If it's not a ghost shrimp but something nastier, then I'd maybe ask to be compensated. If it's a ghost shrimp - I really doubt it was to blame for those fins.